An internal combustion engine is a heat engine in which the burning of a fuel such as hydrocarbons occurs in a confined space called a combustion chamber. This exothermic reaction of a fuel with an oxidizer creates gases of high temperature and pressure, which are permitted to expand. The defining feature of an internal combustion engine is that useful work is performed by the expanding hot gases acting directly to cause movement, for example by acting on pistons, rotors, or even by pressing on and moving the entire engine itself. One of the most common internal combustion engines in automobiles are piston engines.
Valves are used in most piston engines to open and close the intake and exhaust ports in the cylinder head. The valve is usually a flat disk of metal with a long rod known as the valve stem extending from one end. The stem is used to push down on the valve and open it, with a spring generally used to close it when the stem is not being pushed on. Desmodromic valves are closed by positive mechanical action instead of by a spring, and are used in some high speed motorcycle and auto racing engines, eliminating ‘valve float’ at high RPM.
The power output of the engine is dependent, at least in part, on the ability of the engine to allow large volume flow of both air-fuel mixture and exhaust gas through the respective valve ports, typically located in the cylinder head. Therefore a great number of resources are used in designing this part of an engine. Factory flow specifications are generally lower than what the engine is capable of, but due to the time-consuming and expensive nature of smoothing the entire intake and exhaust track, compromises in flow for reduction in cost is often made. In order to gain power, irregularities such as casting flaws are removed and with the aid of a flow bench, the radii of valve port turns and valve seat configuration can be modified to promote high flow. This process is called porting, and can be done by hand, or via CNC machine.
There are many common design and porting strategies to increase flow. Increasing the diameter of the valves to take up as much of the cylinder diameter as possible to increase the flow into the intake and exhaust ports is one method. However, increased valve size can increase valve shrouding, i.e., the impedance of flow created by the cylinder wall. To counteract this adverse effect, valves are commonly designed to open into the middle of the cylinder, such as the Dodge Hemi or the Ford Cleveland engines with canted valves. Also, increasing valve lift, or the distance valves are opened into the cylinder or using multiple smaller valves can increase flow. With the advent of computer technology, in modern engines valves events can be controlled directly by the engines computer, optimizing engine operation at any speed or load. Atomization is conversion of bulk liquid into a spray or mist, i.e., collection of drops, often by passing the liquid through a nozzle.
An atomizer is an atomization apparatus—carburetors, airbrushes, misters, and spray bottles are only a few examples of atomizers used ubiquitously. In internal combustion engines, fine-grained fuel atomization is instrumental to efficient combustion.